A metal oxide semiconductor (MOS) transistor includes source/drain regions formed in a semiconductor substrate, and an oxide layer and a gate poly formed on the semiconductor substrate.
The MOS transistor can be classified into an N type metal oxide semiconductor (NMOS) transistor or a P type metal oxide semiconductor (PMOS) transistor according to channel type. A complementary metal oxide semiconductor (CMOS) transistor includes an NMOS transistor and a PMOS transistor formed on the same substrate.
The CMOS transistor is used to form a circuit requiring a pair of PMOS and NMOS transistors, such as an inverter and a flip-flop circuit.
Recent MOS transistors have fine dimensions so as to satisfy the need of high-performance and highly-integrated devices. However, since a power voltage to the MOS transistor is not changed, the intensity of an electric field in the MOS transistor is high. That is, since the current MOS transistors have a narrow gate, an electric field is concentrated onto a drain. As a result of the electric field concentration, carriers of a depletion layer adjacent to the drain receive energy from the intensive electric field, and thus undesired effects such as a hot carrier effect occur.
Therefore, a CMOS transistor having a lightly doped drain (LDD) structure is introduced to prevent the hot carrier effect. The LDD type CMOS transistor includes lightly and gradually doped source/drain regions formed between a drain and a channel.
Owing to the LDD structure, the intensity of an electric field can be reduced, and the electric field can be distributed in a drain direction so as to reduce an undesired current in a substrate and deterioration of a device.
However, many processes such as the LDD region forming process, a cleaning process, a process of forming a cap oxide layer, and a process of forming lateral spacers are required to form the LDD structure in which lightly and gradually doped source/drain regions are formed between a drain and a channel to prevent the hot carrier effect.